As is known in the art, large mainframe computer systems and data servers sometimes require large capacity data storage systems. One type of data storage system is a magnetic disk storage system. Here a bank of disk drives and the computer systems and data servers are coupled together through an interface. The interface includes storage processors that operate in such a way that they are transparent to the computer. That is, data is stored in, and retrieved from, the bank of disk drives in such a way that the mainframe computer system or data server merely thinks it is operating with one mainframe memory. One type of data storage system is a RAID data storage system. A RAID data storage system includes two or more disk drives in combination for fault tolerance and performance.
As is also known in the art, it is sometimes desirable that the data storage capacity of the data storage system be expandable. More particularly, a customer may initially require a particular data storage capacity. As the customer's business expands, it would be desirable to corresponding expand the data storage capacity of the purchased storage system.
Small Computer Systems Interface (“SCSI”) is a set of American National Standards Institute (“ANSI”) standard electronic interface specifications that allow, for example, computers to communicate with peripheral hardware.
SCSI interface transports and commands are used to interconnect networks of storage devices with processing devices, e.g., using serial SCSI transport media and protocols such as Serial Attached SCSI (“SAS”).
A typical data storage system includes storage processing circuitry and an array of disk drives. The storage processing circuitry stores data into and retrieves data from the array of disk drives on behalf of external host computers. In some conventional data storage systems, the storage processing circuitry includes a Serial Attached SCSI interface (SAS) integrated circuit (IC) which communicates with the array of disk drives using the SAS protocol. The SAS IC enables the storage processing circuitry to operate as a SAS initiator by providing SAS commands to the array of disk drives. The array of disk drives operates as a set of SAS targets by responding to the SAS commands (e.g., by reading and writing data in response to the SAS commands). A typical data storage system may also include a SAS expander integrated circuit (IC). The SAS expander IC acts as a router allowing one SAS port's communications to be selectively routed to a number of different SAS targets.
One conventional data storage system includes two storage processors for high availability. Each storage processor includes a respective SAS IC having a send port and a receive port for each disk drive. Accordingly, if one storage processor fails, the other storage processor has access to each disk drive and can attempt to continue operation.
In the above-described conventional data storage system, each storage processor further includes a parallel bus device which is separate from the SAS IC of that storage processor. A direct memory access (DMA) engine of each storage processor then engages in DMA-based store and retrieve operations through the parallel bus devices to form a cache mirroring interface (CMI) path between the storage processors. As a result, each storage processor is capable of mirroring data in the cache of the other storage processor. With data mirrored in the caches, the storage processors are capable of operating in a write-back manner for improved response time (i.e., the storage processors are capable of committing to data storage operations as soon as the data is mirrored in both caches since the data remains available even if one storage processor fails).